Feature - Add support for relationships

[SoftWyer]

This change adds support for handling relationships in both Android and iOS (mentioned in #54 )

Also:

• Updated full example to be NNBD and bumped dependencies
• Fix tests on Windows due to CRLF parsing when unfolding VCF

I didn't update the API docs, as it adds 300+ changes

It's no problem if you don't want to integrate this - thought I'd add back as I need this in my app.

(I'll be adding Custom Field support next, should be done in a few days)

[joachim-quis]

Important Notice: Repository Reset for v2.0

This repository is being reset for a complete v2.0 rewrite of flutter_contacts. The existing v1 codebase and full git history will be preserved in the legacy_v1 branch.

What this means for your PR:

• This PR will become unmergeable after the reset
• If your change is a feature or fix that's still relevant for v2, please review the new codebase after the migration and consider re-implementing it against the new main branch

Note that v2 is a complete rewrite with many new features including full macOS support, comprehensive vCard import/export, SIM contacts, contact groups, account management, blocked numbers, ringtone assignment, and more. This PR may already be addressed in v2.

Thank you for your contribution, and apologies for the inconvenience!